Photodetectors that convert a light signal into an electrical signal have wide applications in light signal detection. As an emerging candidate for next‐generation light sensing, organic photodetectors compensate well for the shortages of the traditional inorganic photodetectors in terms of ease of processing, compatibility with flexible substrates, tunable absorption characteristics, low‐cost manufacturing, and being lightweight. Currently, regular improvements in organic photodetectors are made with respect to their important figure‐of‐merit performances, which have caught up to or even surpassed the performances of inorganic Si and Ge‐based photodetectors. Importantly, the spectral response of organic photodetectors covers a wide range of wavelengths, from the ultraviolet to near‐infrared regions, with low‐band‐gap organic semiconductors as the active medium. In this paper, the working mechanism and recent advances in organic semiconductor photodetectors are comprehensively reviewed and the challenges in the field, mainly focusing on the performance of organic photodetectors, studies oriented toward applications, and the expectations of organic semiconductor photodetectors in the future, are disclosed.

By using mixed hosts with bipolar transport properties for blue emissive layers, a novel phosphorescence/fluorescence hybrid white OLED without using an interlayer between the fluorescent and phosphorescent regions is demonstrated. The peak EQE of the device is 19.0% and remains as high as 17.0% at the practical brightness of 1000 cd m(-2) .

Tailor‐made red thermally activated delayed fluorescence (TADF) molecules comprised of an electron‐withdrawing pyrazino[2,3‐f][1,10]phenanthroline‐2,3‐dicarbonitrile core and various electron‐donating triarylamines are developed. They can form intramolecular hydrogen‐bonding, which is conducive to improving emission efficiency and promoting horizontal orientation and show near infrared (NIR) emissions (692–710 nm) in neat films and red delayed fluorescence (606–630 nm) with high photoluminescence quantum yields (73–90%) in doped films. They prefer horizontal orientation with large horizontal dipole ratios in films, rendering high optical out‐coupling factors (0.39–0.41). Their non‐doped OLEDs exhibit NIR lights (716–748 nm) with maximum external quantum efficiencies (ηext,max) of 1.0–1.9%. And their doped OLEDs radiate red lights (606–648 nm) and achieve record‐beating ηext,max of up to 31.5%. These new red TADF materials should have great potentials in display and lighting devices.

Thermally activated delayed fluorescence (TADF) materials with through‐space charge transfers (CT) have attracted particularly interest recently. However, the slow reverse intersystem crossing (RISC) and radiative decay always limit their electroluminescence performances. Herein, TADF molecules with ortho‐linked multiple donors‐acceptor (ortho‐Dn‐A) motif are developed to create near‐degenerate excited states for the reinforcement of spin‐orbit coupling. The incorporation of both through‐bond and through‐space CT enlarges oscillator strength. The optimal ortho‐D3‐A compound exhibits a photoluminescence quantum yield of ca. 100 %, a high RISC rate of 2.57×106 s−1 and a high radiative decay rate of 1.00×107 s−1 simultaneously. With this compound as the sensitizer, a TADF‐sensitized‐fluorescent organic light‐emitting diode shows a maximum external quantum efficiency of 31.6 % with an ultrapure green Commission Internationale de L'Eclairage y coordinate value of 0.69.

COMMUNICATIONbetter organic photodetector performance could be achieved if a low dark current density and a high EQE can be reached at the same time.In this paper, we present the fabrication and characterization of an all polymer photodetector sensing from 300 nm to 1000 nm with a calculated peak detectivity greater than 1.0 × 10 13 Jones in the NIR region, on the basis of the shot noise limit. The device also shows a respectable EQE of 27.7% at a wavelength of 850 nm under −0.5 V bias. Importantly, the dark current density is as extremely low as 0.64 nA cm −2 , which should be the best result among all wide spectrum response organic photodetectors reported so far. It can be seen that introducing a thin cross-linkable hole transporting/electron blocking layer greatly reduces the dark current density, while simultaneously preserving the photoresponse.A low-bandgap diketopyrrolopyrrole (DPP)-based polymer poly(diketopyrrolopyrrole-terthiophene) (PDPP3T), as shown in Figure 1 a, is utilized as the donor material, which has proven to be a promising candidate for organic fi eld-effect transistors (OFETs) and organic solar cells. [21][22][23][24] This polymer possesses a narrow-bandgap of 1.3 eV and nearly balanced hole and electron mobilities of 0.04 and 0.01 cm 2 V −1 s −1 . [ 21 ] As shown, it is able to cover the UV/visible/NIR spectral region and outputs a high photoresponse when incorporated with a (6,6)-phenyl-C71-butyric acid methyl ester (PC 71 BM) acceptor.Poly[ N , N ′-bis(4-butylphenyl)-N , N ′-bis(phenyl)-benzidine] (poly-TPD) is an effi cient hole transport material with a hole mobility of about 2.0 × 10 −3 cm 2 V −1 s −1 and has been widely used in polymer/quantum dot light emitting diodes as hole transporting layer due to its resistance to nonpolar organic solvents, such as toluene and p-xylene. [25][26][27] As reported, poly-TPD is also cross-linkable under 254 nm UV light exposure, [ 28 ] which is favorable to multilayer solution processing even with polar solvent, such as chloroform (CF) and o-dichlorobenzene ( o-DCB). In our devices, the cross-linked polymer is used as a buffer layer to collect the photogenerated holes and block the electron injection from the indium tin oxide (ITO) anode to the active layer under reverse bias.To verify the resistance of cross-linked poly-TPD fi lm to polar solvent, the absorption measurements were carried out for the poly-TPD fi lms deposited on fused silica substrates, both with and without UV irradiation. Prior to testing, the fi lms were washed by spin coating with o-DCB solvent and then dried on a hotplate. As shown in Figure 2 , after washing, the absorption remains as high as 92% for the UV light treated fi lm, while the value for the untreated fi lm is only 9.7%. The results suggest that poly-TPD fi lm gets cross-linked and shows excellent polar solvent resistivity after UV light exposure.

Significant increase of photocurrent upon UV light exposure is demonstrated in a narrow‐bandgap polymer‐based photodetector using ZnO nanoparticles as anode interfacial layer. The phenomenon is attributed to the UV light illumination induced oxygen molecules desorption from surface of ZnO nanoparticles, which reduces the electron injection barrier at the anode interface. Ultrahigh external quantum efficiency of 140 000% and extremely low gain threshold voltage of 1.5 mV are achieved in this device with 30 s UV light irradiation. The gain mechanism is explained by the fast transit and replenishment of photogenerated electrons within their lifetime, which is prolonged by the electron‐only device structure, and the experiment results fit well with the proposed photoconductive model.

wileyonlinelibrary.compast few years, hybrid WOLEDs, combining the blue fl uorophors and longwavelength phosphors, have attracted substantial attention owing to the unique merits of high effi ciency and excellent stability. [ 2 ] In principle, to achieve a theoretical maximum internal quantum efficiency for hybrid WOLEDs, a prerequisite key is that all electrically generated singlet and triplet excitons must be effectively utilized for the white emission. [ 2,3 ] Enormous efforts have been devoted to simultaneously harvest both the singlet and triplet excitons in single-emissive-layer (single-EML), [ 4 ] and multi-emissive-layer (multi-EML) hybrid WOLEDs. [ 5 ] In the single-EML hybrid WOLEDs, the precise manipulation of phosphorescent emitter concentration in blue fl uorophore host is very necessary to suppress the singlet exciton transfer from the blue fl uorophore to the phosphors via Förster energy transfer. In this case, the phosphorescent dopant concentration and the property of used blue fl uorophore host have obvious effects on the device performance and there exist the problems of notorious spectrum shift with the increased operational voltages. Alternatively, the multi-EML counterparts provide a reliable strategy Thermally activated delayed fl uorescence (TADF)-based white organic lightemitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior effi ciency/color rendering index/ color stability and low effi ciency roll-off simultaneously. In this paper, highperformance hybrid WOLEDs are demonstrated by employing an effi cient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum effi ciency, current effi ciency, and power effi ciency of 23.0%, 51.0 cd A −1 , and 51.7 lm W −1 , respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m −2 . The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue-green-red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high-performance hybrid WOLEDs.

Herein, we report a deep‐red TADF emitter pCNQ–TPA, composed of quinoxaline‐5,8‐dicarbonitrile (pCNQ) acceptor and triphenylamine (TPA) donor. pCNQ–TPA supported its OLED with desired CIE coordinates of (0.69, 0.31) and the record maximum external quantum efficiency of 30.3 %, which is the best red TADF diode with Rec.2020 gamut for UHDTV. It is showed that through tuning pCNQ–TPA doping concentration, intra‐ and inter‐molecular charge transfer are balanced to synchronously improve emission color saturation and TADF radiation, and remedy aggregation‐induced quenching, rendering photoluminescence quantum yield (PLQY) reaching 90 % for deep‐red emission peaked at ≈690 nm. Quasi‐planar structure further endows pCNQ–TPA with an improved horizontal ratio of emitting dipole orientation, which increases light out‐coupling ratio to 0.34 for achieving the state‐of‐the‐art device efficiencies.